Optimized truss foundations, adapters for optimized truss foundations and related systems and methods

ABSTRACT

An A-frame shaped truss foundation system for a single-axis tracker with moderately sloped legs that translate lateral loads into tension and compression without substantially increasing the magnitude of the lateral load force while optimizing material usage. Several such truss foundation systems are installed in a row to support a torque tube of a single-axis solar tracker. An adapter joins ends of adjacent upper legs and separates the truss legs by an angle more than 35-degrees up to 70-degrees. In some cases, the adapter may have an integrated bearing.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. utility patent application Ser. No.16/413,552 filed May 15, 2019, now U.S. Pat. No. 10,670,303, entitled“OPTIMIZED TRUSS FOUNDATIONS, ADAPTERS FOR OPTIMIZED TRUSS FOUNDATIONS,AND RELATED SYSTEMS AND METHODS,” which claims priority to U.S.provisional patent application No. 62/745,188 filed on Oct. 12, 2018,titled “OPTIMIZED A-FRAME FOUNDATIONS FOR AXIAL SOLAR ARRAYS AND RELATEDSYSTEMS AND METHODS”, the disclosures of which are hereby incorporatedby reference in their entirety.

BACKGROUND

Solar energy is one of Earth's largest potential sources of energy.Above the atmosphere, solar irradiance per unit area is 1.361 kilowattsper square meter. At sea level, the usable energy density is reduced to250 watts per square meter. Using a two-dimensional model to approximatethe Earth, 250 watts/square meter*π*6,371,000 meters² yields about32,000 terra (trillion) watts of energy that continuously strikesEarth's surface. Assuming the sun continues to burn and emit photons fora billion more years, the survival of human life ultimately depends onharnessing this essentially unlimited, source of clean energy.

The main impediment to widescale solar adoption thus far has been cost.Unlike other energy sources, solar energy costs are frontloaded whilethe operating costs are comparatively low. Fossil fuel-based energysources require up-front costs as well as pay-as-you-go costs fromconsuming fuel. Unfortunately, not all the ongoing costs are reflectedin the price of energy generated from fossil-fuel sources. These “dirty”energy sources have significant external costs stemming from CO₂emissions that, in the absence of a carbon tax, are not reflected in thecost. In addition to this cost advantage, entrenched utilities andfossil fuel producers have lobbied effectively to stymie the progress ofsolar, even in states with the greatest solar potential.

Notwithstanding these headwinds, the cost of solar has now dropped lowenough that even when coupled with energy storage, solar power isequivalent to or less expensive than power generated from coal, oil andeven natural gas. In the context of the electricity market, the relativecost difference between competing sources is quantified in terms of thecost per unit, typically a kilowatt hour (kWh). Large scale solararrays, so called “utility-scale” arrays, may have tens to hundreds ofmegawatts of power generating capacity, putting them on the same scaleas small coal and natural gas-fueled power plants. These arrays generatepower that is fed into the grid and sold at wholesale prices on theorder of a few cents per kWh.

The development of utility-scale solar projects is typically fundedagainst power purchase agreements (PPAs). With a PPA, an off-taker(e.g., utility, grid operator, etc.) agrees to purchase all the powergenerated by the system at a fixed rate for the operational life of thearray (e.g., 30 years). This enables a bank or other investor toaccurately value the future revenue stream and to loan money against itto finance construction of the array.

Utility-scale solar power plants are predominantly configured asfixed-tilt ground mounted arrays or single-axis trackers. Fixed-tiltarrays are arranged in East-West oriented rows of panels tilted South atan angle dictated by the latitude of the array site—the further awayfrom the equator, the steeper the tilt angle. By contrast, single-axistrackers are installed in North-South rows with the solar panelsattached to a rotating axis called a torque tube that moves the panelsfrom an East-facing orientation to a West-facing orientation throughoutthe course of each day, following the sun's progression through the sky.For purposes of this disclosure, both fixed-tilt and single-axistrackers are referred to collectively as axial solar arrays.

Excluding land acquisitions costs, overall costs for buildingutility-scale arrays include site preparation (road building, leveling,grid and water connections etc.), foundations, tracker or fixed-tilthardware, solar panels, inverters and electrical connections (conduit,wiring, trenching, grid interface, etc.). Many of these have come downin price over the past few years due to ongoing innovation and economiesof scale, however, one area that has been largely ignored isfoundations. Foundations provide a uniform structural interface thatcouples the system to the ground. When installing a conventionalsingle-axis tracker, after the site has been prepared, plumb monopilesare usually driven into the ground at regular intervals dictated by thetracker manufacturer and/or site plan; the tracker system components aresubsequently attached to the head of those piles. Most often, thesemonopiles have an H-shaped profile, but they may also be C-shaped oreven box-shaped. In conventional, large-scale single-axis trackerarrays, the procurement and construction of the foundations mayrepresent up to 5-10 percent of the total system cost. Despite thisrelatively small share of the total cost, any savings in steel and laborassociated with foundations will amount to a significant amount of moneyover a large portfolio of solar projects. Also, tracker developmentdeals are often locked-in a year or more before the installation costsare actually incurred, so any post-deal foundation savings that can berealized will be on top of the profits already factored intocalculations that supported the construction of the project.

One reason monopiles continue to dominate the market for single-axistracker foundations is simplicity. It is relatively easy to drivemonopiles into the ground along a straight line with existing technologyeven though their design is inherently wasteful. The physics of amonopile mandates that it be oversized because single structural membersare not good at resisting bending forces. When used to support asingle-axis tracker, the largest forces on the foundation are not fromthe weight of the components, but rather the combined lateral force ofwind striking the solar panels. This lateral force gets translated intothe foundation as a bending moment. The magnitude of the bending momentis much greater than the static loading attributable to the weight ofthe panels and tracker components. It acts like a lever arm trying tobend the pile, and the longer the lever arm, the greater the magnitudeof the force. Many tracker companies specify a minimum foundation heightof 40-inches or more. Therefore, in the context of single-axis trackers,monopile foundations must be oversized and driven deeply into the groundto withstand lateral loads.

One proposed alternative to monopile foundations is to use a pair ofsteeply angled legs to form an A-frame or truss-like foundation. AnA-frame has the advantage of converting lateral loads into axial forcesof tension and compression in the legs. As an example, this is seen inpublished U.S. Patent Application, 2018/0051915 (herein after, “the '915application”). The '915 application teaches a support device for solarpanels that consists of a pair of ground screws driven into the groundeither parallel or at steep angles to one another and joined aboveground with a bridge. According to the disclosure, in the angledembodiments, the legs are inclined towards one another at an angle thatis preferably between 10 and 35-degrees, and more preferably between 15and 25-degrees. That angle is the separation of the legs at the apex ofthe A-frame and corresponds to a leg angle in a range of ±72.5-degreesto ±85-degrees and more preferably ±78.5-degrees to ±82.5-degrees withrespect to horizontal. As discussed in greater detail herein, such steepangles, while still capable of translating lateral loads into tensionand compression, will result in tensile and compressive forces muchlarger than the underlying lateral load. The magnitude of the tensileand compressive forces generated by lateral loads is non-linearlycorrelated to leg angle, a fact that is not recognized by the teachingof the '915 application. As a result, at such steep angles, the legsmust be oversized or include additional orthogonal features to resistthe large values of tension and compression that are generated.Otherwise, the foundation will fail. This is part of the reason whyground screw-based A-frames have failed to gain traction in the utilityscale solar industry, other than in the most difficult soils wherecostly refusals dominate.

In recognition of this problem, it is an object of various embodimentsof this disclosure to provide a truss or A-frame foundation forsingle-axis trackers that is limited to a range of angles that reducesthe non-linear magnitude of tensile and compressive forces imparted tothe truss from lateral loads and thereby optimizes the amount of steeland depth of embedment needed for a given diameter leg.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a portion of an exemplary single-axis tracker systemsupported by multiple conventional monopile foundations;

FIG. 1B is an end view of the system of FIG. 1A showing the range ofangles of the solar panels as the torque tube rotates;

FIG. 1C is a force diagram showing how lateral loads are translated intoa bending moment in a conventional monopile foundation of FIG. 1A;

FIG. 2A shows a portion of an exemplary single-axis tracker systemsupported by a moderately sloped A-frame foundation;

FIG. 2B is an end view of the system of FIG. 2A showing the range ofangles of the solar panels as the torque tube rotates;

FIG. 2C is a force diagram showing how lateral loads are translated intotension and compression in the moderately sloped A-frame foundation ofFIG. 2A;

FIG. 3A shows a portion of a single-axis tracker system supported by aconventional steeply sloped A-frame foundation;

FIG. 3B is a force diagram showing the magnitude of tensile andcompressive forces in the steeply sloped A-frame of 3A at 77.5-degreesin response to a 2,500-pound lateral load;

FIG. 3C is a force diagram showing the magnitude of tensile andcompressive forces in the steeply sloped A-frame of 3A at 82.5-degreesin response to a 2,500-pound lateral load;

FIG. 3D is a graph of tensile and compressive forces through a range ofsteep angles for a 2,500-pound lateral load;

FIG. 4A is a moderately sloped A-frame foundation system according tovarious embodiments of the invention;

FIG. 4B is a force diagram showing the magnitude of tensile andcompressive forces in the moderately sloped A-frame of 2A and 2B at±60-degree in response to a 2,500-pound lateral load;

FIG. 4C is a force diagram showing the magnitude of tensile andcompressive forces in the moderately sloped A-frame of 2A and 2B at±65-degrees in response to a 2,500-pound lateral load;

FIG. 4D is a graph of tensile and compressive forces through a range ofmoderately sloped and steep angles in response to a 2,500-pound lateralload;

FIG. 5A is a diagram showing the correlation between truss angle andA-frame leg length at ±60-degrees.

FIG. 5B is a diagram showing the correlation between truss angle andA-frame leg length at ±70-degrees;

FIG. 5C is a graph showing leg length versus truss leg angle for a rangeof acute angles;

FIG. 6A is a perspective view of a portion of a single-axis tracker andA-frame foundation according to various embodiments of the invention;

FIG. 6B is an end view of the portion of single-axis tracker and A-framefoundation shown in FIG. 6A;

FIG. 7A is a perspective view of a further adapter for a top-downsingle-axis tracker that orients lateral loads at the work point of anA-frame-shaped truss foundation according to various embodiments of theinvention;

FIG. 7B is an end view of the adapter for the top-down single-axistracker of FIG. 7A supporting a single-axis tracker;

FIG. 8 is an end view of a bearing adapter supporting a top-downsingle-axis tracker according to various embodiments of the invention;

FIG. 9 is an end view of a bearing adapter supporting a top-downsingle-axis tracker according to various embodiments of the invention;

FIG. 10 is an end view of a bearing adapter supporting a bottom-upsingle-axis tracker according to various embodiments of the invention;and

FIGS. 11A and 11B are perspective and top views respectively of aconventional single-axis tracker.

DETAILED DESCRIPTION

The following description is intended to convey a thorough understandingof the embodiments described by providing a number of specificembodiments and details involving A-frame foundations used to supportsingle-axis solar trackers. It should be appreciated, however, that thepresent invention is not limited to these specific embodiments anddetails, which are exemplary only. It is further understood that onepossessing ordinary skill in the art in light of known systems andmethods, would appreciate the use of the invention for its intendedpurpose.

As discussed in the background, when a single-axis tracker is supportedby monopile foundations, lateral loads induced by wind striking thearray generate large bending moments that must be resisted by thefoundation. FIG. 1A shows a portion of single-axis tracker 100 supportedby a series of aligned monopile foundations 100. Exemplary tracker 100in this figure consists of solar panels 140, attached to torque tube 130via panel brackets 135. Torque tube 130 is captured within multiplealigned cylindrical bearing assemblies 120 that are each attached atoprespective H-piles 110. Though not shown, one pile will also typicallysupport a drive motor or gear assembly that moves the torque tube. Also,electrical interconnections between the solar panels have beenintentionally omitted. In real world conditions, the panels would beconnected serially to form high voltage direct current (DC) strings thatare fed into one or more combiner boxes and/or inverters.

Single-axis tracker 100 shown in FIG. 1 is a conventional bottom-updesign where the bearing assembly is attached the head of the pile andthe torque tube rotates within the bearing about its own main axis.Other tracker systems may employ a top-down design where the torque tubehangs from a hinge received in the bearing housing so that the torquetube can sweep through an arc like a pendulum. In such systems, thedrive motor is offset from the torque tube's main axis so that thetracker's axis of rotation is about the hinge point. The bearing housingis still attached to the head of each pile, but the axis of rotation isoffset from the torque tube's main axis. One such top-down tracker isdisclosed, for example, in U.S. Pat. No. 10,222,446, which is herebyincorporated by reference in its entirety. The various embodiments ofthe invention are compatible with bottom-up as well as top-down oroff-set single-axis trackers.

FIG. 1B is an end view of the portion of the conventional tracker shownin FIG. 1A. The Figure shows foundation pile 110, bearing assembly 120and torque tube 130. Rotation of torque tube 130 causes solar panels 140to sweep through a range of angles from East to West shown by the dottedlines in the figure. The extent of angular rotation is specified by thetracker maker but currently as much as ±55-degrees to ±60-degrees withrespect to horizontal (0-degrees). Because the pile is plumb (i.e.,oriented at 90-degrees), there is ample clearance for the panels toachieve their maximum rotation angle without mechanical interferencefrom the foundation.

FIG. 1C is a force diagram showing the effect of lateral loads on such atracker. Lateral load F_(L) imparts a bending moment on the foundationat the point where the torque tube is attached to the pile (e.g., at thebearing assembly). The magnitude of the resulting moment M is equivalentto the magnitude of the force F_(L) multiplied by the height H above thepoint where the foundation is pinned to the ground. If the pile extendsfour feet above ground and the lateral load is 2,500 pounds, theresultant moment M will be at least 2500×4 or 10,000 lb./ft. The pinpoint will often be more than one foot below ground because the toplayer of soil provides little resistance to moments so the safety factorof the pile must be increased to account for that reality. Therefore, toachieve the necessary structural rating, the piles used to support thearray must be rated to resist that moment and driven deep enough to keepit pinned when subjected to such a moment. This requires the use of astrong monopile, such as a the commonly used W6×9 or W6×12 H-pile, whichhave a six-inch flange and nine or twelve pounds of steel per linearfoot respectively, with as much as five to seven feet of below groundembedment.

The inventors and applicant of this disclosure have proposed analternative to plumb monopile foundations that aims to significantlyreduce the total amount of steel required to support axial solar arrays.This alternative foundation system, referred to commercially asEARTHTRUSS™, consists of a pair of moderately sloped, substantiallyaligned legs configured as an A-frame or truss that extend above andbelow ground and are joined at the apex with an adapter, bearingassembly, or other torque tube support element. The truss architectureoffers several advantages over conventional monopiles foundations.First, if properly designed, the A-frame will translate lateral loadsinto axial forces of tension and compression in the legs rather thanbending. Individual structural members are poor at resisting bending butrelatively good at resisting axial forces. The A-frame or truss takesadvantage of this by directing those forces along the axis of the legswhere it is best applied. Therefore, the size and gauge of the steelthat makes up the legs may be much smaller than an equivalent monopile.Also, without needing to resist bending, the legs do not need to bedriven as deep as a conventional monopiles. This saves steel but alsoreduces the likelihood of encountering a refusal. A refusal occurs whenadditional impacts of a pile driver fail to result in additionalembedment of the pile. Usually, this is the result of striking rock orcementious soil and requires an expensive, labor-intensive mitigationprocess. The shallower piles are driven, the less likely it is that theywill encounter rock or cementious soil.

Turning now to FIG. 2A, this figure shows a single-axis tracker likethat of FIG. 1A, except that the monopile foundations have been replacedwith multiple aligned A-frame or truss foundations 200. Each trussfoundation 200 consists of a pair of adjacent legs 210 driven into theground at angles to each other so that they are substantially alignedwith each other. In various embodiments, the legs may be angledsymmetrically (e.g., ±60-degrees) with respect to horizontal. In othercases, they may be at somewhat different angles due to variations ingrade and terrain and misalignment during driving. Surface variationscan make measurement of the legs with respect to grade or a horizontalreference difficult. Therefore, throughout the disclosure, A-frame ortruss foundations are characterized in terms of their top angle, eventhough this implies a corresponding leg angle.

In various embodiments, legs 210 may be beaten into the ground with apile driver, rotated into the ground with a rotary driver, pushed intothe ground, or otherwise driven. Also, legs 210 may consist of a singlemember or multiple interconnected members extending along asubstantially common axis. Truss foundations 200 are installed along aNorth-South row at regular intervals specified by the tracker maker. Inthis exemplary system, the above ground ends of each leg 210 are joinedby adapter 220. As shown, bearing assembly 230 sits atop adapter 220.Torque tube 130 passes orthogonally through each bearing assembly 230.It should be appreciated that in other embodiments, and as discussed ingreater detail herein, the bearing assembly and adapter may be combinedinto a single structure to reduce part count, cost and for otherreasons.

FIG. 2B is an end view of tracker system 200 of FIG. 2A. As seen in thisfigure, clearance between the panels and the foundation is tighter thanin the case of the monopile array of FIGS. 1A and B due to angled legs210. If the tracker can orient the panels all the way down to±60-degrees, the legs may have to be separated by a smaller top angle toprevent interference. It is not uncommon for the angle of panels on atorque tube to vary along its length by as much as 5-degrees due tobending and manufacturing tolerances, so an additional buffer of5-degrees may be necessary to avoid interference.

FIG. 2C is a force diagram showing how lateral loads are translated in atruss or A-frame foundation. As discussed herein, the truss architecturedirects lateral forces along the main axis of each structural memberinstead of trying to bend them. In general, the column loading capacityof the legs is much greater than the tensile and compressive forcesgenerated by lateral loads so loading capacity is not the limitingfactor. Rather, a combination of skin friction between the leg surfaceand the soil and resistance from orthogonal threads or otherbelow-ground features will control the truss' ability to resist lateralloads. If designed properly, the primary failure mode should be pullingup on the windward leg and sinking of the leeward leg. As seen in 2C,when lateral load F_(L) strikes the array, the forces are translatedinto the truss legs as tension and compression labeled T and C. For thetruss not to fail, the sum of all forces must be zero. In this case,that means that the sum of forces in the direction of the lateral load,labeled X direction in the figure, must be zero. In other words, theX-component of the resistance provided by the legs to the lateral load,labeled as F_(T/C), must be equivalent to the lateral load F_(L). Theactual value of these forces will also be impacted by the vertical loadfrom the weight of the tracker system, which will resist tension in thewindward leg and add to compression in the leeward leg, however, forpurpose of angle comparison that load has been ignored. The X-componentof resistive forces F_(T/C) is proportional to the cosine of the trussleg angle θ by equation (1):F _(L)=2F _(T/C) cos(Ø)  (1)

Therefore, the required resistive force for each leg F_(T/C) in responseto F_(L) is equal to

$\frac{F_{L}}{2{\cos(\varnothing)}}.$As θ approaches 90-degrees, the cos(θ) will approach zero and therequired resistive force F_(T/C) in each leg will approach infinity.

As discussed in the background above, the '915 application teaches oneexample of an A-frame or truss foundation used to support a single-axistracker. FIG. 3A is a reproduction of the foundation described in the'915 application. System 300 consists of a pair of angled legs 310, inthis case ground screws, joined above ground by a two-piece bridge thatclamps down on the end of each leg. A conventional bearing assembly 330is bolted onto the bridge to complete the assembly. The disclosuredescribes a range for the top angle α between each leg that ispreferably between 10 and 35-degrees, and more preferably between 15 and25-degrees. These ranges for the angle α correspond to truss legs thatare angled in a range of ±72.5-degrees to ±85-degrees (assuming they aresymmetric with respect to horizontal), and more preferably truss legangles θ of ±77.5-degrees to ±82.5-degrees. FIGS. 3B and 3C are forcediagrams showing the magnitude of the required F_(T/C) at the steep legangles and corresponding small apex angles suggested in the '915application for a 2,500-pound lateral load. Starting with 3B, the chosenangle α is 25-degrees corresponding to a leg angle θ=77.5-degrees, thehigh end of the preferred range for α specified in the '915 application.The resistive force required in each leg F_(T/C) is equal to 5,775pounds or 2500/2*Cosine (77.5). Although still under the column capacityof the leg, this results in substantially increased axial forces to beresisted with skin friction and orthogonal threads. In FIG. 3C, the apexangle α is set to 15-degrees, corresponding to a leg angle θ of±82.5-degrees, the lower end of the '915 application's preferred rangefor α. At this angle, assuming the same 2,500-pound lateral load,F_(T/C) increase to 9,576 pounds or 2500/2*Cosine (82.5), nearly afour-fold increase. Table 1 below shows the resistive force required ineach leg (F_(T/C)) for the entire range of angles proposed in the '915application. FIG. 3D is a graph of the same. The first column of Table 1is the truss leg angle θ and ranges from 72.5-degrees to 82.5-degrees inhalf-degree increments. The second column is the top angle α between thelegs when they are joined at the apex for each given leg angle andchanges in one-degree increments. The third column is the resultanttensile and compressive forces F_(T/C) that must be generated to resistthe 2,500-pound lateral load.

TABLE 1 Resistive force for range of angles in ′915 application. Angle ΘAngle α Force F_(T/C) (lbs.) 72.5 35 4157 73 34 4275 73.5 33 4401 74 324535 74.5 31 4677 75 30 4830 75.5 29 4992 76 28 5167 76.5 27 5355 77 265557 77.5 25 5775 78 24 6012 78.5 23 6270 79 22 6551 79.5 21 6859 80 207198 80.5 19 7574 81 18 7991 81.5 17 8457 82 16 8982 82.5 15 9577As Table 1 and the corresponding graph at FIG. 3D show, the resultantforce increases in a non-linear fashion in the range for a proposed inthe '915 application, negating the benefits of using an A-frame or trussfoundation. To resist the large tensile and compressive forces at theseshallow apex angles (steep leg angles), the truss components must beoverbuilt, driven deeply and/or include additional orthogonal features.This will undermine the advantages that can be provided by replacingmonopiles with A-frames by making them relatively more expensive. Thoughnot stated, module interference with the legs may be one reason for therecommended range of angles in the '915 application.

Turning now to FIG. 4A, this figure shows a truss foundation 200 for asingle axis tracker according to various exemplary embodiments of theinvention. The exemplary truss shown here consists of adjacent legs 210driven into the ground to be substantially aligned with each other andalong the torque tube. The significance of this is that if the legs arenot substantially aligned, that is their respective axes don't extend tosubstantially the same place on the torque tube, lateral loads willintroduce a twisting moment to the foundation in addition to tension andcompression. This twisting moment will require additional reinforcing ofthe apex hardware to withstand these twisting forces.

This is seen in the prior art, for example, in U.S. Pat. No. 9,207,000(hereinafter, the '000 patent). FIGS. 11A and B, are reproduced from the'000 patent. Tracker 350 has an A-frame-shaped foundation with legs355A/B supporting torque tube 370. The bearing assembly consists ofrespective bearings 360A/B welded to the top of each leg 355A/B to makethem into unitary structures. The bearings are nested one within theother along torque tube 370 resulting in an offset geometry as seen inthe overhead view of 11B. Because legs 355A/B are offset, that is notsubstantially aligned with respect to the torque tube, or symmetric withrespect to each other, lateral loads will tend to twist the legs,bearings and torque tube. Tracker 350 compensates for this by nestingone bearing within the other, however, this requires a double thicknessof metal at the bearing and that the legs be thicker and/or heavier thanthey would need to be if no twisting moment was generated.

Returning to the system of 4A, in this system, legs 210 are joined atthe top by adapter 220 so that they are substantially aligned withrespect to each other and the torque tube. Bearing assembly 230 sitsatop adapter 220. As a result of the legs alignment, lateral loads willresult in axial forces only and no twisting moment. Also, in contrast totruss 300 of FIG. 3A, truss 200 of this figure is optimized to an anglethat significantly limits the tensile and compressive forces, in thiscase a 60-degree apex angle α and 60-degree leg angle θ. The dramaticeffects of this are shown in the force diagram of 4B. Assuming the same2,500-pound lateral load as in the force diagrams of FIGS. 3B and 3C,the forces F_(T/C) in each leg drop to 2,500 pounds at 60-degrees, or2500/2*cos(60), a 57-percent reduction over the 25-degree apex angle(77.5-degree truss leg angle) of FIG. 3B, and a staggering 74-percentreduction over the 15-degree apex angle (82.5-degree truss leg angle) ofFIG. 3C.

FIG. 4C is force diagram repeating the analysis of FIG. 4B for a30-degree apex angle (70-degree truss leg angle). Assuming the same2,500-pound lateral load, with the legs oriented at 70-degrees, thevalue of the resistive force required in each leg (F_(T/C)) increases to3654 pounds, or 2500/2*cos(70), a 37-percent reduction over the25-degree apex angle (77.5-degree truss angle) of FIG. 3B and a62-percent reduction over the 15-degree apex angle (82.5-degree trussangle) shown in FIG. 3C. Table 2 below shows the resistive forcerequired in each leg (F_(T/C)) for the entire range of angles proposedby the inventors of this application. FIG. 4D is a graph of the of thedata in Table 2 as well as the data in Table 1. The first column is thetruss leg angle θ and ranges from 55-degrees to 72.5-degrees inhalf-degree increments. The second column is the apex or top anglebetween the legs, α, and the third column is the resultant tensile andcompressive forces F_(T/C) that must be generated to resist the2,500-pound lateral load.

TABLE 2 Resistive force for range of angles according to embodiments ofthe invention. Angle Θ Angle α Force F_(T/C) (lbs.) 55 70 2179 55.5 692207 56 68 2235 56.5 67 2265 57 66 2295 57.5 65 2326 58 64 2359 58.5 632392 59 62 2427 59.5 61 2463 60 60 2500 60.5 59 2538 61 58 2578 61.5 572620 62 56 2663 62.5 55 2707 63 54 2753 63.5 53 2801 64 52 2851 64.5 512904 65 50 2958 65.5 49 3014 66 48 3073 66.5 47 3135 67 46 3199 67.5 453266 68 44 3337 68.5 43 3411 69 42 3488 69.5 41 3569 70 40 3655 70.5 393745 71 38 3839 71.5 37 3939 72 36 4045 72.5 35 4157Table 2 and the corresponding combination graph at FIG. 4D show that theresultant force increases in a non-linear fashion. The range of anglesproposed in the '915 application massively increase the requiredresistive forces, negating the benefits of using an A-frame or trussfoundation. The horizontal line drawn at 4000 pounds in FIG. 4D showsthe range of leg angles required to limit F_(T/C) to 4000 pounds orless. To stay below 4000 pounds, the leg angle θ must below 72.5 degreesand the apex angle α above 35-degrees. For apex angles below that, ortruss leg angles above that, the tensile and compressive forces begin toincrease sharply, which has significant cost-impacting consequences forthe truss design. Therefore, the preferred apex angle range is 35-degreeto 70-degrees corresponding to a leg angle below 72.5 degrees. The idealapex angle is above 49-degrees corresponding to a leg angle below 65.5degrees. Angles in this ranges will keep the resultant forces below 3000pounds.

Another consideration in angle selection is material usage. For a givenfixed depth of embedment, steeper angles will require less material.Turning to FIGS. 5A and 5B, these figures present two scenarios thatseek to achieve a four-foot apex height, labeled H in the figures, andthree feet of embedment for each truss leg. By setting the opposite sideof the truss triangle to 4, the desired height of the apex above ground,the hypotenuse (L₂) may be solved for using equation (2):L ₂=4/Sin(θ)  (2)For the 60-degree apex and truss leg angle assumption in FIG. 5A, thisresults in a value for L₂ of 4.62 feet. Adding 3 feet for the desiredembedment length, requires a leg that is 7.62 feet or approximately 1935mm long for a 60-degree truss. In FIG. 5B, the apex angle is decreasedto 40-degrees and the truss leg angle increased to 70-degrees. In thiscase, L₂ drops to 4.26 feet, 4/Sin (70), resulting in a required leglength of 7.26 feet or 1844 mm. Therefore, as between the two, steeperangles allow for shorter legs to be used, reducing the amount of steelrequired. However, as with the required resistive forces, the rate ofchange of leg length with decreasing angle is not linear, and as notedabove, steeper leg angles (narrower apex angles) may require greaterlength of embedment for each leg due to the exponentially increasingforces F_(T/C). Table 3 below shows the pile length for various leg andapex angles assuming a four-foot apex height and three feet of embeddedlength.

TABLE 3 Leg length as a function of truss angle. Angle α Angle Θ LegLength 170 5 48.89 160 10 26.04 150 15 18.45 140 20 14.70 130 25 12.46120 30 11.00 110 35 9.97 100 40 9.22 90 45 8.66 80 50 8.22 70 55 7.88 6060 7.62 50 65 7.41 40 70 7.26 30 75 7.14 20 80 7.06 10 85 7.02 0 90 7.00As the leg angle increases and the apex angle decreases, the leg lengthgoes down to the minimum, the straight-line length of 7-feet where thetruss becomes a monopile. As the leg angle is reduced and apex angleincreased, the leg length begins to increase slightly and then takes offfor angles below 40-degrees. For nearly all leg angles above 40-degrees(apex angles below 100), there is relatively little variance in thelength, ≈2 feet or 29-percent above the minimum of 7-feet. For legangles of 55-degrees of greater, apex angles of 70-degrees or less,there is less than one foot of variance, or 14-percent longer than theminimum. Therefore, the preferred leg angle from a material usageperspective is one that is at least 40-degrees and ideally 55-degrees orgreater, corresponding to a preferred apex angle less than 100-degrees,and ideally less than 70-degrees.

Taken together, the information in Tables 1, 2, and 3 and FIGS. 3D, 4D,and 5D show that there are optimal ranges for α and θ that balancecompeting goals of minimizing F_(T/C) against minimizing material usage.To the first point, the preferred apex angle α is more than 35-degreesup to 70-degrees, while the ideal range is from 55-60-degrees,corresponding to a preferred truss leg angle θ from 55 to less than72.5-degrees, with an ideal range from 60 to 65-degrees. From a materialusage perspective, any angle α below 100-degrees or angle θ above40-degrees is acceptable but angles for a below 70-degrees and angles θabove 55-degrees are preferred. These ranges overlap with the optimalrange of the apex angles α and truss leg angles θ to minimizing F_(T/C).

Referring now to FIGS. 6A and 6B, these figures show a single-axistracker array and an A-frame foundation for a single-axis tracker arrayor other axial solar array according to various embodiments of theinvention. As discussed above in the context of FIGS. 2A and 2B, onepotential disadvantage of an A-frame type foundation relative to aconventional H-pile is that as the torque tube rotates the panels intosteep angles (greater than ±50 degrees), the legs of the A-frames mayinterfere with the panels. This could require using a smaller apex angleor steeper leg angle for the A-frame, as taught in the '915 application,but this is not optimal due to the rapidly and non-linearly increasingvalue of F_(T/C). Another mitigation technique is to limit the range ofmotion for the tracker, but this too is untenable because it eliminatesthe ability to harvest so-called “shoulder power” obtained at thebeginning and end of each day, when the sun is rising from or settingtowards the horizon. Tracker manufacturers design their systems toharvest shoulder power and advertise this feature, so limiting rotationof the tracker is impractical from a business perspective.

The inventors of this disclosure have proposed a solution that overcomesthis problem without limiting the torque tube's range of rotation. Thesolution introduces a gap, such as gap 145 shown in FIG. 6A, at eachpoint on the torque tube above one of the A-frame foundations (e.g., atthe torque tube bearing locations). Because the legs of each trussdefine a substantially common East-West oriented plane, the amount ofspacing required is little more than the outside diameter of the legsthemselves or the thickness of the bearing assembly. In this way, thetorque tube can rotate through a full range of angles (up to and beyondthe leg angle) and is limited only by the length of slack wireinterconnecting the modules to non-rotating components. As seen, in 6B,with a 60-degree truss leg and apex angle, the torque tube can rotatethe panels all the way to 70-degrees or even steeper withoutinterference.

The remaining FIGS. 7A-B, 8, 9, and 10 show various truss adapters andbearing adapters that separate the legs by an apex angle in the desiredrange of more than 35-degrees up to 70-degrees according to variousembodiments of the invention. Starting with FIGS. 7A-B, these figuresshow adapter 400 with a main body portion 410, planar bearing supports412, and a pair of aligned connecting portions 415 extending down andaway from main body portion 410 to couple to truss legs 210. Theconnecting portions 415 are preferably symmetric, meaning that straightlines through their respective centers will intersect at a common point.Adapter 400 could support a bottom-up or top-down style tracker. 7Bshows adapter 400 attached to upper legs 212 via connecting portions415. In various embodiments connecting portions 415 are oriented so thatupper legs 212 are angled apart from each other at the apex at an anglein a range of more than 35-degrees up to 70-degrees. In variousembodiments, fitment between connecting portions 415 and upper legs 212will allow for adjustment between these structures to enable theadapter's height relative to upper legs 212 to be adjusted beforepermanently connecting it to upper legs 212. The dotted lines extendingthrough the center of each leg 210 show that connecting portions 415 arealigned in the vertical direction. Though not shown, a top view wouldalso show that the lines through their respective centers intersecting,showing that they are also horizontally aligned, i.e., intersecting atsubstantially the same point along the torque tube. The tracker shown in7B is a top-down tracker, such as that shown in FIGS. 2C and D.

FIG. 8 shows cardioid-shaped bearing adapter 500 specifically adaptedfor a top-down single-axis tracker. As used in this disclosure andclaims, the term bearing adapter is used generically to refer tostructures that combine the function of an adapter, i.e., joining thelegs of the A-frame so that they are aligned and at the correct angle,and the functions of a bearing assembly, i.e., supporting the torquetube as well as providing a cylindrical bearing that receives arotational member that serves as the tracker's axis of rotation. Bearingadapters are not limited to only bottom-up or only top-down trackers,and instead may accommodate either, as is shown and discussed in greaterdetail below. Bearing adapter 500 of FIG. 8 performs at least threefunctions.

First, it unifies the adjacent legs of the A-frame to form a rigid trussstructure. Second, it provides a cylindrical bearing opening for abearing pin to hang the torque tube from and clearance for it to swingthrough its East-West arc while reducing the steel and components neededto do so. Finally, it aligns upper legs 212 so that their respectiveaxes are aligned with respect to each other and the torque tube sets theapex angle between them in a range of greater than 35-degrees up to70-degrees, and ideally in a range of 50-degrees to 60-degrees.

Bearing adapter 500 is formed from one or more cast metal pieces. Itshould be appreciated, however, that other manufacturing techniques maybe employed without departing from the spirit or scope of the invention.It consists of cardioid-shaped hoop 510 with cusp 512 and symmetricS-shaped arms 520, located on either side of cusp 512. S-shaped arms 520terminate in respective tubular connecting portions 517. S-shaped arms520 project away from cusp 515, then curve back inwards before curvingback out to match the angle and spacing of the truss legs (e.g.,±60-degrees). In various embodiments, the desired angle truss leg angleθ and apex angle α will be known in advance and the bearing adapter 500will be manufactured to set that angle.

Connecting portions 517 shown in the example of these figures are drawnas partial tubes. In various embodiments, separate cover plates that arealso tubular may fit over the upper legs 212 of the A-frame and matewith connecting portions 517 to capture the upper legs. Bolts, rivets,or other known mechanical fasteners and their functional equivalents maybe used to secure the cover plates to connecting portions 517. It shouldbe appreciated that other embodiments may have a complete tube at theend of each S-shaped arm 520 that is dimensioned to receive or fitinside the free ends of each adjacent upper leg 212, or a completelydifferent geometry. Such modifications are within the scope of thevarious embodiments of the invention as long as they don't undermine thefunctions of the adapter. A cylindrical bearing opening in cusp 512receives the bearing pin from which torque tube 130 is suspended. Modulebrackets 135 couple the solar panels (photovoltaic modules) to torquetube 130. Because a monopile foundation is not used, the ends ofS-shaped members 520 do not need to intersect at a horizontal mountingplatform. This design takes more efficient advantage of the trussarchitecture and may simplify installation. For example, if after eachpair of adjacent truss legs is installed, the torque tube or torque tubesections may be laid on the ground between the adjacent unconnectedlegs. Once bearing adapters, such as adapter 500, are attached to torquetube 130 at the appropriate locations, the entire tube or tube sectionscan be lifted with a forklift or other equipment so that individualconnections between adjacent legs and their respective bearing adapterscan be completed.

Turning now to FIG. 9, this figure shows an end view of a top-downsingle-axis tracker and bearing adapter 600 for such a tracker thatjoins the truss to form a A-frame foundation and aligns the legs so thatthey are separated from each other at the apex by an angle α in a rangeof more than 35-degrees up to 70-degrees, and ideally in a range of 50to 60-degrees. The top-down tracker could be a mechanically balancedtracker system such as that available from NEXTRACKER or other adifferent top-down tracker from another tracker maker. Bearing adapter600 provides the functionality of conventional torque tube clampsupports (e.g., elements 124/126 in FIGS. 2C/2D) but in a form factoroptimized for truss foundations.

Starting with the foundation, as with those shown in conjunction withthe other truss adapters and bearing adapters discussed herein, itconsists of legs 210 each formed from screw anchors 211 and upper legs212. Screw anchors 211 have been rotated into the underlying soil atreciprocal angles with respect to horizontal, in this example±68-degrees so that they are separated at the top by an angle α of44-degrees. In various embodiments, each screw anchor 211 has a threadform at its distal end to help it resist axial forces and keep it ontrack during driving. The thread form may be uniformly wide or may havea tapered lead-in. Upper leg 212 is axially coupled to the above-groundend of each screw anchor 211 to form a partial A-frame using one of thevarious joining methods discussed herein.

Bearing adapter 600 is like bearing adapter 500 of FIG. 8 but is tubularrather than cast. Connecting portions 615 are connected to respectiveends of each upper leg 212 to complete the A-frame. In variousembodiments, a collar, pin, coupler, crimp or their functionalequivalents may be used to join the second connecting portion 615 to thesecond upper leg 212 after the first connecting portion 615 has beenconnected to the first upper leg 212. The hinge and clamp assemblyholding the torque tube are supported by bearing adapter 600 via bearingpin 620 extending through the cylindrical bearing opening in bridgesection 610. As in other top-down systems shown herein, the clampassembly holds the torque tube and suspends it so that it can rotateabout bearing pin 620. In addition, in this system a pair of U-boltsstraddle the torque tube to pin the module support bracket to torquetube 130. Photovoltaic modules, otherwise known as solar panels, arethen attached along the torque tube 130 to each module support bracket,with each bracket joining the edges of two adjacent modules. In variousembodiments, the connection between bearing adapter 600 and upper legs212 will allow for some relative height and angular adjustment. Invarious embodiments, installation of adapter 600 shown in FIG. 9 may belike that described in the context of bearing adapter 500 of FIG. 8.

Turning now to FIG. 10, this figure shows bearing adapter 700 accordingto various other embodiments of the invention. Unlike the bearingadapters shown in FIGS. 8 and 9, adapter 700 is designed for a bottom-upstyle of single-axis tracker. Functionally it is equivalent to adapter500/600 because it aligns and connects the truss legs to complete theA-frame, provides a cylindrical bearing for receiving a rotating member,in this case the torque tube, and sets the apex angle to the desiredrange. However, the geometry of adapter 700 is different because thebearing receives the torque tube rather than a hinge and therefore noclearance is required for the torque tube to swing through an arc.

Bearing adapter 700 consists of lower body portion 710 with alignedconnecting portions 715 projecting down and away from lower body portion710 to connect to upper legs 212. As with the adapters of various otherembodiments, connecting portions 715 are angled down and away from lowerbody portion 710 so that the truss legs will be separated from eachother by an angle α of more than 35-degrees to 70-degrees, andpreferably at about 50 to 60-degrees. Lower body portion 710 has asemicircular cross section because it serves as the lower half of acylindrical bearing. Upper portion 720 fits over lower body portion 710via upper flanges that sit on corresponding lower flanges in lower bodyportion 710. Bolts, screws, or other known fasteners or their functionalequivalents may secure upper portion 720 to lower body portion 710. Inthis example, torque tube 730 is shown with a boxed cross section. It issurrounded by bearing insert 725 which, translates the boxed tube to acircular cross-sectional shape to match the profile of the cylindricalbearing formed from the union of upper portion 720 and lower portion710. Bearing adapter 700 is optimized from a material usage perspectiveto work with an A-frame-shaped truss foundation by joining the legs ofthe A-frame, keeping them at the desired angle and spacing, and aligningthem with respect each other. It also provides a cylindrical bearing sothat a separate bearing assembly is not needed.

The embodiments of the present inventions are not to be limited in scopeby the specific embodiments described herein. Indeed, variousmodifications of the embodiments of the present inventions, in additionto those described herein, will be apparent to those of ordinary skillin the art from the foregoing description and accompanying drawings.Thus, such modifications are intended to fall within the scope of thefollowing appended claims. Further, although some of the embodiments ofthe present invention have been described herein in the context of aparticular implementation in a particular environment for a particularpurpose, those of ordinary skill in the art will recognize that itsusefulness is not limited thereto and that the embodiments of thepresent inventions can be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breath and spirit ofthe embodiments of the present inventions as disclosed herein.

The invention claimed is:
 1. A truss foundation system for single-axistrackers comprising: a pair of truss legs driven partially intounderlying; and an adapter for joining the pair of truss legs to form atruss foundation with the ground, wherein the adapter comprises a mainbody portion, a bearing support portion above the main body portion forsupporting a tracker bearing assembly, and a pair of connecting portionsextending away from the main body portion for connecting to respectiveones of the truss legs so that the pair of truss legs are separated byan apex angle α, where 35-degrees <α≤70-degrees.
 2. The truss foundationsystem according to claim 1, further comprising a bearing assemblyhaving a cylindrical bearing portion for receiving a rotating member. 3.The truss foundation system according to claim 2, wherein the bearingassembly is built into the adapter.
 4. The truss foundation systemaccording to claim 3, further comprising a bearing insert in thecylindrical bearing portion.
 5. The truss foundation system according toclaim 2, wherein the bearing assembly is attached to and supported bythe adapter.
 6. The truss foundation system according to claim 2,wherein the rotating member is a tracker torque tube.
 7. The trussfoundation system according to claim 2, wherein the rotating member is abearing pin.
 8. The truss foundation system according to claim 7,further comprising a torque tube suspended from the bearing pin.
 9. Asingle-axis tracker comprising: a torque tube; a plurality of solarpanels attached to the torque tube; a plurality of bearing assembliessupporting the torque tube; a plurality of plurality of trussfoundations wherein each truss foundation comprises a pair of adjacentlegs extending into underlying ground; and a plurality of adaptersjoining each pair of adjacent truss legs, each adapter supporting one ofthe bearing assemblies and having a pair of connecting portions thatseparate each truss leg of the pair by an apex angle α, where 35-degrees<α≤70-degrees.
 10. The single-axis tracker according to claim 9, whereinthe bearing assemblies and adapters are components of a singlestructure.
 11. The single-axis tracker according to claim 9, wherein thebearing assemblies are attached to and supported by respective ones ofthe adapters.
 12. The single-axis tracker according to claim 9, furthercomprising a gap between adjacent solar panels greater than a width ofeach adapter at least location along the torque tube supported by one ofthe truss foundation, the gap preventing mechanical interference betweenthe plurality of solar panels and plurality of truss foundations. 13.The single-axis tracker according to claim 9, wherein each bearingassembly comprises a bearing supporting the torque tube so that thetorque tube may rotate about its own axis.
 14. The single-axis trackeraccording to claim 9, wherein each bearing assembly comprises a bearingsupporting a bearing pin from which the torque tube is suspended.
 15. Anadapter for joining a pair of legs of a truss foundation systemcomprising: a bridge portion having a bearing that defines a rotationalaxis that is transverse to a width of the bridge portion; a rotatingmember seated in the bearing and a pair of connecting portions forjoining the pair of legs to form a truss foundation so that the pair oftruss legs are separated by an apex angle α, where 35-degrees<α≤70-degrees, wherein the pair of connection portions are substantiallyaligned about opposing sides of the rotational axis.
 16. The adapteraccording to claim 15, wherein the rotating member comprises a bearingpin seated in the bearing.
 17. The adapter according to claim 16,further comprising a section of torque tube suspended from the bearingpin.
 18. The adapter according to claim 15, wherein the connectingportions extends from respective sides of the bridge portion.